Last year Ronaldo signed a five-year contract to keep him at Real until 2018 but he refused to rule out a second spell in England.

"I love Manchester, and to be back, I never know, you never know. [In] football you never know," he told national newspapers. "Of course, I'm happy at Real Madrid, it's my home, it's my club, but Manchester treated me unbelievable so we never know."

Ronaldo joined United from Sporting Lisbon for 12.8 million pounds in 2003 and left for a then world record fee of 80 million pounds in 2009.

He scored 118 goals in 292 games for the English club, winning three Premier League titles and the Champions League in 2008.

Cristiano Ronaldo was worshipped by the Old Trafford faithful during a hugely successful six-year spell at Old Trafford.

He was granted a rapturous reception at Old Trafford when Real faced United in the Champions League in 2013 and looked back fondly on his time in England, adding: "It was one of my favourite clubs, Manchester. Everyone knows that every time when I speak about Real Madrid I should speak about Manchester United, I loved it there.

"I was 18 years old [when I joined], I win everything there, the people treat me like a God there so I appreciate that and the future, nobody knows. I remember when we played there against Manchester in the Champions League: they kill me, my heart was so happy.<

"I had fantastic moments and I appreciate this club. For me, it's one of the best clubs in the world and I have passion for this club. Some players I am still in contact with because when I was there, I created a good relationship with everybody, not just the players, but all the staff around.